An audio DAC with internal DSP
core has been designed that includes three high-quality audio DACs as well as a
DSP programmed to provide algorithms that correct for speaker/amplifier
deficiencies. These algorithms include equalization, two-band
compression/limiting with arbitrary compression curve, spatial enhancement,
delay and interpolation. All signal processing parameters are
register-programmable using an external microcontroller. The DAC architecture
uses a multi-bit sigma-delta design with a mismatch-shaping scrambler.

Contrary to the MPEG-1 Audio
compression schemes, Advanced Audio Coding (AAC) in its MPEG-2 and MPEG-4 flavours
has no inherent upper limit for the sampling frequency of the input signal.
Furthermore, the bitstream format allows to cover a dynamic range far beyond
the range provided by 24 bit linear PCM coding. This makes the AAC coder an
ideal candidate for representing audio signals with parameters that are usually
associated with high resolution audio systems. This paper discusses the
application of this highly efficient compression scheme to digital programme
material represented with 24 bit and 96 kHz.

Stanley P. Lipshitz and
John Vanderkooy, University of Waterloo, Waterloo, Ontario, Canada

This paper extends our previous
studies of 1-bit sigma-delta modulators. We now investigate 1l⁄2 bit
(i.e., 3 level) systems, and again find that we can predict their idle-tone
behavior and the spectral structure of their output. We address the question of
whether sigma-delta modulators are adequately dithered by their internal noise,
and compare the behavior of minimum-phase and nonminimum-phase (i.e., chaotic)
modulator designs. Numerous computer simulations, greatly aided by coherent
averaging, have guided us to two basic mechanisms that explain the distortion:
the sweeping of the idle tone, and the saturating quantizer characteristic. A
model of the nonlinearity of a dithered quantizer explains the general nature
of the harmonic distortion components and their phases. The data also show
unambiguously that the maximum possible dither should always be used.

Convention Paper 5477

3:30 pm

N-4 Effective Dither in High-Order
Sigma-Delta Modulators

This paper clarifies some of the
cofusion that has arisen over the efficacy of dither in PCM and Sigma-Delta
modulation systems. It describes a fair means of comparison between them. It
presents results that show dither is effective in sigma-delta modulation
systems of any order and proposes methods for achieving optimum performance in
both systems.

A noninvasive method of audio
file identification is described to ascertain the proximity of processed audio,
of possibly dubious origin, to a reference file. A differential correction
function forms the basis of a comparative metric. Testing embraces linear and
nonlinear processes including perceptual-based codecs. Applications include
DVD-A and SACD when used as the only legitimate nonwatermarked release formats.